Parts such as stiffened composite structures (e.g., stringers) formed from laminated composites may be fabricated using a flat multi-ply charge of prepreg materials. The charge may be formed into a desired part shape using a forming press having mating dies that compress and form the charge into the desired shape, which is normally a straight or smoothly contoured shape with a longitudinal ridge. However, using a forming press may present difficulties when attempting to form joggles in the composite structure because of the tendency of the plies to form wrinkles as the charge is being compressed near a joggle detail, in particular when that joggle detail crosses the longitudinal ridge. Unchecked wrinkles may necessitate repairing, reworking, and/or scrapping the composite structure.
A joggle is a local offset in the global contour of a surface, i.e., the global contours on either side of a joggle are approximately parallel. A joggle includes two bends, or changes in local contour, that are opposite each other and relatively close to each other. For example, in a sheet-like structure, a joggle is an offset bend consisting essentially of two opposite bends, each less than 90°, separated by a neutral web.
Present solutions to avoid wrinkles in joggled, stiffened composite structures include adding pre-cure patches to reinforce wrinkled sections and using hand lay-up techniques in which each ply is laid up by hand over a die or other tool in order to reduce the possibility of wrinkling. These techniques are material intensive (and thus costly and result in heavy parts), labor intensive (and thus costly), and/or relatively slow.
A further problem with current techniques for forming joggled, stiffened composite structures is the limited flexibility of existing forming machines to accommodate differing part shapes that are joggled. Since the tooling normally has a permanent shape that may not be easily altered, separate tooling dies must be fabricated to accommodate differing part shapes (e.g., different positions and bends along a structural member).
Accordingly, there is a need for methods and apparatuses for forming joggled, stiffened composite structures that reduce and/or eliminate wrinkling during the forming process. There is also a need for apparatuses for forming joggles in stiffened composite structures that are easily reconfigurable to form parts having differing shapes.